For many years it has been recognized that blood pressure measurement provides a very useful indication of the relative health of a patient, especially in traumatic situations. Typically, emergency medical personnel find it desirable to monitor such things as a patient's blood pressure, respiration, capillary refill rate, dilation of the pupils, and temperature in traumatic situations such as on a battlefield, in an emergency vehicle en route to a hospital after an auto accident, or the like. Blood pressure measurement is very difficult under these circumstances using conventional non-invasive techniques.
The most common blood pressure measurement technique involves listening to the patient's pulse with a stethoscope while varying the pressure in an occlusion bladder placed over a limb, typically the arm. Initially, the pressure in the occlusion bladder is increased until blood flow stops. That is, the pressure in the occlusion bladder is raised above the systolic pressure. The bladder pressure is then gradually reduced over a few seconds, while the patient's pulse is monitored. The characteristic sounds of pulses in the bloodstream are referred to as "Korotkoff noises." When a first such noise is detected, the pressure in the bladder at that time corresponds to the systolic pressure, the pressure exerted by the heart during its contraction, when blood is pumped from the chambers of the heart. A subsequent change in the characteristic sound of the pulse, when the Korotkoff noises cease, indicates that the pressure in the bladder is equal to the diastolic pressure, the pressure exerted by the heart on the blood vessels during the heart's dilation, when the chambers of the heart relax and refill with blood. To audibly detect pulses, i.e. to hear the Korotkoff noises, requires a relatively quiet environment. Accordingly, blood pressure measurement techniques using stethoscopes to detect these sounds are best performed in a hospital or similar environment. They are particularly unsuitable for use in an emergency vehicle, on a battlefield, or the like, where the noise level is very high.
It will be appreciated by those skilled in the art that "noise" in this context includes audible noise, which interferes with the ability of the physician or emergency medical personnel to hear the Korotkoff noises in the stethoscope, as well as physical motion of the patient either with respect to the physician or during transport in a vehicle. Such motion, of course, generates additional variations in pressure which can mask or obscure pulses in the blood pressure, or which can sound to the physician like Korotkoff noises, thus causing an inaccurate measurement to be made.
Accordingly it is an object of the invention to provide a blood pressure measurement system which does not require a physician to listen to a patient's pulse through a stethoscope or the like.
There have been developed a large number of methods and apparatus for mechanized measurement of a patient's blood pressure. Some of these are invasive, i.e. involve insertion of a catheter or the like into an artery. Such invasive methods are undesirable for obvious reasons. All of the non-invasive measurement techniques of which the present inventors are aware, mechanized or not, involve detection of the pulses caused by the pumping action of the heart. Whether detected by a physician using a stethoscope or by a machine, detection of pulses is rendered difficult or impossible by noise, again including physical motion of the patient.
Accordingly, it is an object of the invention to provide a method of measuring the blood pressure of a patient which does not involve detection of the pulses caused by the pumping action of the heart, so as to realize substantial noise immunity.
A number of patents have been issued on automated devices for measuring the blood pressure of a patient. One such device is taught in U.S. Pat. No. 4,437,470 to Prost. Prost teaches controlling the blood pressure in a patient's finger by means of an occluding cuff and monitoring the transparency of the skin area downstream of the occluding cuff. The occluding cuff pressure is gradually varied over a period of on the order of 20 seconds, so that blood flow is at first interrupted and is then gradually resumed. The instrument comprises means for monitoring the pressure as a function of time and identifying certain inflection points in the transparency curve as indicating the systolic and diastolic pressure. In particular, Prost teaches an empirical formula for combining pressure measurements which he states has been experimentally determined to provide a more accurate measure of the systolic pressure than does any particular one of the inflection points.
Prost, however, still requires that the pulse be detected (column 5, lines 14-22, and claim 4). Accordingly, the Prost device is not suitable for use in a noisy environment such as an emergency vehicle or on a battlefield.
The Croslin et al. U.S. Pat. No. 3,581,734 shows an apparatus for measuring blood pressure comprising a larger cuff for occluding the brachial artery and a smaller cuff located on the forearm for continuously detecting the magnitude of blood pulses. A pressure transducer on the upper cuff provides measurement of systolic and diastolic pressures. The technique employed requires detection of the pulse, which is performed by the smaller cuff which controls the indicating circuitry.
The McNally et al. U.S. Pat. No. 4,080,966 shows an appartus and method for regulating blood pressure in which the mean arterial pressure is determined in a conventional manner, by filtering the blood pressure, which is measured using a conventional directly invasive technique. The filtering is preferably accomplished by integration.
The Doll U.S. Pat. No. 4,134,396 covers a method for measuring steady flow of blood using a non-invasive flow meter which relies on electronic methods for detecting pulses in the flow.
The Hood, Jr. U.S. Pat. No. 4,461,266 shows a device for measuring blood pressure which relies on determination of mean arterial pressure. The Hood device also requires detection of the pulse.
It is a further object of the invention to provide a non-invasive blood pressure measurement technique which provides accurate values for diastolic and systolic pressure without need of detection of the pulses in the blood flow, which is suitable for a noisy environment, which is simple and relatively inexpensive, and which is adaptable to blood pressure sensing without removal of the patient's garments, including relatively heavy uniforms or oversuits, so as to be useful in a wide variety of conditions and circumstances.
Classically, the systolic and diastolic pressures have been considered the most relevant information concerning a person's blood pressure. More recently, however, the mean arterial pressure, that is, the mean value for the blood pressure, has become of increased interest to some medical personnel, typically anesthesiologists and the like. Accordingly, it is an object of the invention to provide an instrument which provides accurate values for the systolic, diastolic and mean arterial blood pressures of a patient, and which employs a non-invasive technique which does not require detection of pulses in the blood flow.